Stylus profilers are used for obtaining surface profiles of samples. The stylus of the profiler follows the surface under a small contact force, and the resulting motions of the stylus are measured with a sensor assembly. The sensor assembly includes a stylus, a mechanical linkage (usually a stylus arm) connecting the stylus to a flexure pivot, and a transducer. When the stylus is scanned across the surface of the sample, the force exerted by the sample surface on the stylus causes a rotation of the stylus arm about the flexure pivot. The vertical displacement of the stylus is converted by the transducer into an electrical signal which indicates the profile of the sample surface.
Advanced profilers also include a force control mechanism, such as an electromagnetic actuator, for maintaining a constant contact force between the stylus and the sample surface as the stylus is scanned across the surface. To maintain a constant contact force between the stylus and the sample surface, the spring action of the flexure pivot is calibrated and the force control magnetic actuator is controlled to counteract the change in the force applied by the flexure spring on the stylus caused by rotation of the stylus arm. Thus, a constant force is exerted by the stylus against the sample surface, as the stylus is scanned across the surface. As an example of a profiler which has been used in the semiconductor and disk drive industries, please see U.S. Pat. Nos. 5,705,741 and 5,309,755; both patents are incorporated herein in their entirety by reference.
As the semiconductor industry progresses to smaller dimensions with each new generation of products, there is an increasing need for scanning instruments that can measure sub-micrometer scale surface features. While the depths or vertical dimensions (dimensions normal to the plane of the wafer surface) of the features such as trenches, or via holes, in semiconductor wafers, commonly exceed one micrometer, the lateral dimensions (dimensions in the plane of the wafer surface) have been continually reduced. At the current state of the art, the lateral dimensions of features such as trenches are less than 0.5 micrometer. With the continual reduction of the lateral dimensions of features such as trenches and via holes in the surface of semiconductor wafers, the ratio of depth to the lateral dimension of such features, also known as the aspect-ratio, is continually increased.
In order to measure such high aspect-ratio features, a very sharp, thin but long (high aspect-ratio) stylus must be used. However, a sharp, thin but long stylus is fragile and may easily break, especially when subjected to lateral forces (forces in directions in, or parallel to, the plane of the sample surface). Thus, when a high aspect-ratio stylus contacts a steep feature, such as the side wall of a trench or via hole, the contact force has a relatively large lateral component and a relatively small vertical component. Stylus profilers, such as the profilers described in the two patents referenced above, are designed such that motion of the stylus is constrained to one degree of freedom, namely, rotation about the flexure pivot. This degree of freedom is substantially normal to the sample surface. The stylus arm is relatively stiff in all other degrees of freedom. Consequently, the lateral forces generated when the high aspect-ratio stylus encounters a steep wall can easily break the stylus and damage the sample being measured.
The stylus arm in a profiler has a single degree of freedom, which comprises rotations about a pivot. The stylus or sensing tip travels along a path normal to a radial line passing through the center of rotation at the pivot and the tip. Since the sensing or stylus tip must be located xe2x80x9cbelowxe2x80x9d or at a lower elevation than the pivot to ensure that the tip and not the body of the sensor assembly contacts the sample, the motion of the stylus or sensing tip is not truly normal to the plane of the sample surface, but is in the shape of an arc. While the main direction of travel of the tip is downwards, it nevertheless also travels in the lateral direction in the plane of the sample surface. This lateral motion is also known as parasitic motion of the sensing tip. The parasitic motion of the sensing tip may hamper or even preclude the sensor assembly from measuring relatively deep and narrow features.
It is therefore desirable to provide an improved surface measurement system which overcomes the above drawbacks.
The above-described difficulties can be overcome by allowing the sensing tip of the profiler to contact the sample surface without substantially rotating the stylus arm about the pivot. Instead, a distance between the sample and the sensing tip of the profiler is reduced until the tip touches the sample, without moving the tip and the sample laterally relative to each other. By avoiding lateral relative motion between the tip and the sample before the tip touches the surface, the above-described problems are avoided. When such a scanning process is used, thin and long (high aspect-ratio) styli can be used to penetrate high aspect-ratio features for measurement. Data related to the height of the sample may then be measured with the tip stationary and in contact with the sample. After the measurement, the tip and the sample are separated and moved laterally relative to each other to measure the sample surface at a different location.
With minor modifications, the above-described scanning process may also be applied to other scanning instruments, such as the scanning probe microscope, which includes atomic force microscopes and scanning tunneling microscopes.
As described above, the feature on the sample surface may be found and measured by repeatedly causing the sensing tip (of the profiler or scanning probe microscope, for example) and a sample to repeatedly contact at different locations of the sample surface. In this process, the sensing tip and the sample are brought together substantially without lateral relative motion between them until they contact, separated again substantially without lateral relative motion between them, and moved laterally relative to each other until the tip is at a location above a different portion of the sample. This process is repeated at different locations of the sample. If the separation between the tip and the sample during such lateral motion is less than the change in height of the sample surface, the lateral motion will cause the sensing tip to contact the sample surface laterally, thereby causing damage to the sensing tip. To reduce the probability of such damage, the separation may be increased to a large value before lateral relative motion is initiated. If no knowledge of the height variation or distribution of the sample surface is available, such value should be large enough that it exceeds any probable height variations of the sample surface one may encounter. The resulting process can be quite time consuming, especially if the sample surface is to be measured at many different locations. This difficulty can be avoided by separating the tip and the surface by just enough to avoid such lateral contact.
A number of techniques may be employed to assure that the sensing tip and the sample are separated by an adequate distance so that the sensing tip will not contact the sample surface during the subsequent lateral relative motion. In the preferred embodiments, if certain height information is provided concerning the sample surface or a portion thereof (such as within a target area), then the sensing tip may be positioned at or close to the portion of the sample having the highest elevation. If the height information of the sample surface or a portion thereof is not readily available, such information can be acquired quickly by actually measuring the height of the surface at several sampling locations. Yet another technique that can be employed is to actually measure data related to the height of the sample when the tip and the sample come into contact as the tip is scanned across the sample and use such measured data to predict an elevation of the next location of the sample to be measured, so that the separation between the tip and the sample can be set to be higher than such predicted elevation. These are, of course, only some examples of the techniques that can be used to implement the above general concept.
In some applications, it may be desirable to first find the feature quickly, and then take an appropriate amount of time to actually measure the feature. In this instance, the distance between the sensing tip and the sample is controlled so that the distance between the tip and the sample is periodically increased and then decreased as the tip scans across the sample surface until the tip either touches the surface or until either the tip or the surface has traveled, or the two together have traveled in aggregate, by a preset distance without causing the tip and the surface to contact. In other words, the sensing tip does not completely penetrate the feature when scanning across the surface.
For some applications, to save time, even without any prior knowledge concerning the topology of the sample surface, the sample surface can be quickly scanned and measured without incurring undue risk in breaking the sensing tip. This involves determining whether the tip and the surface remain in contact after the distance between them is increased to a predetermined value, before lateral relative motion between them is initiated or continued. If the tip and the surface remain in contact after they are being separated from each other by a predetermined distance, the distance between them is further increased until they are no longer in contact before moving the tip and the surface laterally with respect to each other.
A sensor assembly having a sensing probe may be used in any one of the above-described processes for sensing the sample. The sensor assembly includes a base portion and a moveable sensing tip connected to the base portion. When the tip contacts the sample, the tip may move relative to the base portion of the sensor assembly. A moving stage is used to cause vertical relative motion between the sensor assembly and the sample. Thus, when the moving stage causes a distance between the sensor assembly and a sample to be reduced until the sensing tip contacts the sample, the actual change in distance between the sensing tip and the sample is given by a combination of the relative motion between the sensor assembly and the sample, and of the relative motion between the sensing tip and the base portion. By taking into account both motions, a more accurate measure of data related to the height of the sample can be obtained. In different embodiments, the sensor assembly may be that of a profiler, an atomic force microscope or other types of scanning probe microscopes.
One way to increase measurement speed while avoiding significant lateral forces between the sensing tip and the sample is to separate the process into two parts: an initial fast find mode to find the feature of interest, and after finding the feature, a second measurement mode to measure the feature.
As one possible embodiment of the invention to implement the above two part process, the sensing tip is scanned across the sample surface with the tip in contact with the surface until the feature is found. Scanning the sensing tip across the surface with the tip in contact with the surface speeds up the scanning process. After the feature is found, the sensing tip is scanned across the feature with the tip in intermittent contact with the sample surface to measure the feature. In this manner, the time required to find and measure the feature is reduced without undue risk of large lateral forces between the sensing tip and the sample surface. In the preferred embodiment, two different styli with known tip offsets are used in this process. The first stylus is used to scan while in contact with the surface to find the feature. The second stylus is then used to measure the feature.
In yet another aspect of the invention, a sensor assembly having a sensing probe with a sensing tip is employed. When the sensing tip of the probe is used for sensing a sample, the vertical distance moved by the sensor assembly (or by both the assembly and the surface in aggregate) until the tip contacts the surface within a feature of interest may be taken as the depth of the feature. To determine that the tip has contacted the surface, the sensor assembly is driven towards the surface until the distance moved by the tip relative to the assembly exceeds a threshold, at which point the vertical relative motion between the tip and the surface is stopped and the vertical distance moved by the assembly (or the sum of the distances moved by both the assembly and surface) is noted to indicate the depth of the feature. To yield a more accurate measure of such depth, the motion of the probe tip relative to the assembly is taken into account in calculating such depth.